A chopper system, for example, may be used for power control of a charge storage element when a piezoelectric element is used as the charge storage element and a DC voltage source is used as a power source. The chopper system is used when it is unnecessary to electrically non-insulate the power source side and the load side from each other.
When it is required to arbitrarily control the voltage of the charge storage element on the basis of a condition to be equipped to the system, it is required to step up/down the voltage in some cases. In this case, a circuit system called as “step up/down chopper” is known.
Circuits illustrative of such types of circuit systems are illustrated in, for example, FIGS. 22, 23 and 24. The circuit construction of FIG. 22 includes a DC voltage source 101, a switch 102, a diode 103, an inductor 104, a capacitor 105, an inductor 106, a switch 107, a diode 108 and a piezoelectric element 109. A step-up chopper circuit is constructed by the elements 106, 107, 108, and a step-down chopper circuit is constructed by the elements 102, 103 and 104. The step-up chopper circuit and the step-down chopper circuit are connected to each other through an intermediate capacitor 105 in series. The circuit construction of FIG. 23 is designed so that the step-up chopper circuit and the step-down chopper circuit of FIG. 22 are connected to each other in series in the opposite style to the circuit construction of FIG. 22. The circuit construction of FIG. 24 is designed so that an inductor 110 functions as both the inductor 104 of the step-down chopper circuit of FIG. 22 and the inductor 106 of the step-up chopper circuit of FIG. 22, and thus one of the inductors 104 and 106 and the intermediate capacitor 105 which are needed in the circuit construction of FIG. 22 can be eliminated.
In all the circuit constructions of FIGS. 22 to 24, the polarity of the output voltage is the same as the DC voltage source 101.
If the diodes 103 and 108 are replaced by switches in FIGS. 22 to 24, powering and regeneration can be performed, that is, bi-directional power control can be performed. Furthermore, when MOSFETs are used as these four switches, it is possible to suppress the conduction loss due to synchronous rectification in addition to powering and regeneration.
FIG. 25 shows a circuit construction which is considered as another type of step up/down chopper. The circuit construction of FIG. 25 comprises a DC voltage source 101, a switch 102, an inductor 110, a diode 111 and a piezoelectric element 109. The DC voltage source 101, the switch 102 and the inductor 110 are connected to one another in series, and the inductor 110, the diode 111 and the piezoelectric element 109 are connected to one another in series. In the circuit constructions of FIGS. 22 to 24, two switches and two diodes are needed. However, in the circuit construction of FIG. 25, each number of the switches 102 and the diodes 111 can be reduced to one. Furthermore, in the circuit of FIG. 25, the output voltage has the opposite polarity to that of the DC voltage source 101, and the applied voltage to each of the switch and the diode is increased, so that there occurs no problem even when a high voltage-resistant element is used in the circuit of FIG. 24.
In some types of systems, it is required to not only charge the piezoelectric element 109 (charge storage element), but also discharge the piezoelectric element 109. In order to satisfy this requirement, the diode 111 is replaced by a switch in FIG. 25 to enable the discharging of the piezoelectric element 109. FIG. 26 shows this circuit construction (for example, non-patent document 1). That is, a switch 120 is provided in FIG. 26. In this circuit construction, not only charging of the piezoelectric element 109 (charge storage element), but also power regeneration of the power source can be performed.
[Non-patent document 1] Bhaskar Krishnamachari and Dariusz Czarkowski, “Bidirectional Buck-boost Converter with Variable Output Voltage”, 1998 IEEE International Symposium on Circuits and Systems (ISCAS '98), June 1998.
Miniaturization is required to power converters at all times, and a method of increasing the switching frequency is known as a method of implementing miniaturization of power converters. However, the mere increase of the switching frequency causes a problem that the switching loss is increased and the efficiency of the power converter is reduced.
With respect to the switching loss, a voltage applied to the switch and current flowing in the switch at the switching time are varied, so that a loss (=voltage×current) occurs. Furthermore, at the switching time, the time variation rate of the voltage applied to the switch and the time variation rate of the current flowing in the switch are increased to higher values as compared with the states other than the switching state, so that electromagnetic noise occurs. Such a switching mode is generally referred to as “hard switching”.
Therefore, a soft switching technique is known as a method of reducing the switching loss per pulse. The switching loss per pulse can be reduced, and thus the switching frequency can be increased in the power converter equivalently to the loss, so that the power converter can be miniaturized. As the soft switching technique is applied to a field in which the switching frequency is higher and the switching loss is more predominant, the loss reducing effect or the miniaturization effect is more remarkable.
A method of including a capacitor and an inductor in the main circuit construction and actively using a resonance phenomenon is known as a main method of the soft switching technique. FIG. 27 shows an example of this method. In FIG. 27, a series circuit including a capacitor 130, an inductor 131 and an auxiliary switch 132 is connected to the switch 102 in parallel, and a series circuit including a capacitor 133, an inductor 134 and an auxiliary switch 135 is connected to the switch 120 in parallel. For example, in a case where the switch 102 is turned on to make current flow through the inductor 110 and then the switch 102 is turned off, the capacitor 130 is charged in advance, and the switch 132 is turned on so that current flowing in the opposite direction to the current flowing in the switch 102 just before the switch 102 is turned off is made to flow in the closed circuit comprising the switch 102, the capacitor 130, the inductor 131 and the auxiliary switch 132, thereby carrying out soft switching.
However, the circuit construction of FIG. 27 needs additional elements whose number is larger than the number of the constituent parts of FIG. 26 (excluding the DC voltage source 101, the piezoelectric element 109), and it has a disadvantage that the body size thereof is increased and the cost is also increased. Therefore, with respect to the application of the soft switching technique to a polarity inverting type step up/down chopper in which the voltage of the piezoelectric element 109 (charge storage element) described with reference to FIG. 26 is reduced to zero or less, it is needed to establish a soft switching circuit that is optimal in loss, cost and body size to the step up/down chopper in which the voltage of the piezoelectric element 109 (charge storage element) is reduced to zero or less.